Upgrading of pre-processed used oils

ABSTRACT

A process to further upgrade a pre-processed used lubricating oil by: (a) contacting the partially upgraded used oil in the presence of hydrogen with a hydrodemetallization catalyst; (b) contacting the effluent of step (a) in the presence of hydrogen with a hydrotreating catalyst; (c) contacting the effluent of step (b) in the presence of hydrogen with a dewaxing catalyst; and (d) contacting the effluent of step (c) in the presence of hydrogen with a hydrotreating catalyst.

The invention is directed to a process to further upgrade apre-processed used lubricating oil.

WO-A-9961566 describes a process to prepare a pre-processed used oil byremoval of solids, low boiling compounds and polycyclic compounds fromused oils.

The pre-processed used oils as obtained by such a process cannot bedirectly used as, lubricating base oil to formulate new lubricants.While some properties like the Viscosity Index (VI) generally do complywith industry standards for HVI (High VI) base oils (VI greater or equalto 95), other properties like pour point and Health/Safety/Environment(HSE) characteristics generally do not. It is an object of the presentinvention to provide a process to further upgrade the products asobtainable by the process of WO-A-9961566 or similar pre-processed usedoils.

The following process achieves this object. Process to further upgrade apre-processed used lubricating oil by:

-   (a) contacting the pre-processed used oil in the presence of    hydrogen with a hydrodemetallization catalyst,-   (b) contacting the effluent of step (a) in the presence of hydrogen    with a hydrotreating catalyst, and-   (c) contacting the effluent of step (b) in the presence of hydrogen    with a dewaxing catalyst, and-   (d) contacting the effluent of step (c) in the presence of hydrogen    with a hydrotreating catalyst.

Applicants have found that with the above process excellent quality HVI(high viscosity index) base oils can be obtained from used oils having asufficiently low pour point and excellent characteristics with respectto HSE aspects. The base oils as prepared by this process pass inparticular the General Motors LS/2 suite of health tests (expressed interms of Total PNAs (as measured by EPA SW-846), Residual elements (asmeasured by ASTM D5185), total PCB (as measured by EPA SW-846), totalorganic halogens and Modified Ames Test (as measured by ASTM E 1687)Further advantages of the process will become clear from the belowdescription.

The pre-processed used oil can be prepared from various sources of usedoils. The used oils are suitably subjected to an extraction treatmentwherein most of the additive package resids, water and other insolublesare separated from the oil. The extraction is preferably performed withpropane as the extraction solvent as for example described in U.S. Pat.No. 4,265,734, U.S. Pat. No. 5,286,380 and U.S. Pat. No. 5,556,548.Prior to the extraction process, zinc based additives and degradationproducts can be removed by precipitation as described in for exampleU.S. Pat. No. 4,376,040 and CA-A-2068905. The pre-processed oil may alsobe obtained from used oil by, for example, contacting the used oil witha basic substance and a phase transfer catalyst in the presence ofwater, contacting this mixture with liquid propane, separating theimpurity-free oil from the propane and re-refining said impurity-freeoil. Such a process is for example described in detail in theaforementioned WO-A-9961566.

Suitable pre-processed used oils have an oxygen content of less than 1wt % and more preferably less than 0.5 wt % as calculated as the weightof oxygen atoms in the oil feed. The majority of this oxygen will bepresent as the bound oxygen of water molecules. Furthermore thepre-processed used oil suitably contains less than 2 wt % nitrogen andmore preferably less than 0.05 wt % nitrogen. Furthermore thepre-processed used oil suitably contains less than 2 wt % sulphur andmore preferably less than 1 wt % sulphur. Typical pre-processed usedoils will contain between 10-300 ppm chlorine. For the present processaccording to the invention the upper chlorine content is preferably lessthan 200 ppm and more preferably less than 150 ppm chlorine. The totalcontent of phosphorus, calcium, zinc and silicon is typically between 20and 1000 ppm and preferably between 20 and 300 ppm. Other (metal)compounds, such as iron, and sodium may also be present in lowquantities.

The pour point of the pre-processed oil is preferably below 0° C. Theviscosity index of the pre-processed oil is preferably above 90.

The pre-processed used oil, which is used as feedstock in the presentprocess preferably has an initial boiling point of between 340 and 380°C. and more preferably about 370° C. The boiling point at which 95 vol %(T95) is recovered is preferably between 480 and 550° C. and morepreferably between 500 and 540° C. It has been found that thepre-processed used oils having a higher T95 boiling point will contain ahigh level of compounds such as phosphorus, calcium, zinc and silicon.Such a high level of these compounds is detrimental for the catalystlife in the process according to the present process.

An example of the above described pre-processed oils is the LightDistillate as obtainable from the reclaiming process of InterlineResources Corporation as described in detail on their web pagehttp://www.interline-resources.com/introduction.html as viewed on 1 Aug.2000. The Light Neutral Distillate typically has an Initial BoilingPoint (IBP) of more than 300° C. and preferably more than 340° C., a T50(temperature at which 50 wt % of the distillate is recovered) in therange of between 430-470° C. and a Final Boiling Point (FBP) of below600° C. The above feedstock may be blended with small portions of otherhydrocarbon sources, such as for example the Heavy Neutral Distillate asobtained from the same Interline process. The heavy Neutral Distillatehas typically an IBP of more than 300° C., a T₅₀ of between 500-520° C.and a FBP greater than 650° C.

Suitable hydrodemetallization catalysts to be used in step (a) are forexample the hydrodemetallization (demet) catalysts developed to removenickel, vanadium and molybdenum from crude oil residues. It has beenfound that such catalysts also reduce the content of halogens, such aschlorine and fluorine, but also phosphorus, calcium, zinc and silicon ina sufficient manner under hydro-processing conditions. Examples of suchhydro-demetallization processes are described in U.S. Pat. No. 4,297,242and U.S. Pat. No. 4,613,425. Such catalysts comprise suitably an aluminacarrier, a Group VIB metal and optionally a non-noble Group VIII metal.Optionally phosphorus is deposited on the catalyst. A suitable Group VIBmetal is molybdenum. Suitable non-noble Group VIII metals are nickel andcobalt. The alumina carrier is suitably more porous than the aluminasupport of the hydrotreating catalyst of steps (b) and (d).

In a preferred embodiment step (a) is performed using more than one typeof hydrodemetallisation catalysts wherein the feed is first contactedwith hydrodemetallisation catalysts having a high uptake capacity formetals and then contacted with hydrodemetallisation catalysts having arelatively higher desulphurisation and denitrification activity than thefirst type of catalyst or catalyst combination. Examples of suitablecommercial hydrodemetallization catalysts are RM-430, RN-410 and RN-412as obtained from Criterion Catalyst Company (Houston, US).

The catalyst used in step (a) is preferably presulfided before use(ex-situ and/or in-situ). Presulphiding of the catalyst can be achievedby methods known in the art, such as for instance those methodsdisclosed in the following publications EP-A-181254, EP-A-329499,EP-A-448435, EP-A-564317, WO-A-9302793 and WO-A-9425157.

Step (a) is suitably operated at a temperature of between 330 and 420°C. The pressure may range from 10 to 250 bar, but preferably is between20 and 150 bar. The weight hourly space velocity (WHSV) may range from0.1 to 10 kg of oil per litre of catalyst per hour (kg/l.h) and suitablyis in the range from 2 to 10 and more preferably between 4 and 6 kg/l.has calculated on the total of demet catalyst used in step (a).

In step (b) especially the level of nitrogen is reduced. Thehydrotreating catalyst to be used in step (b) may therefore be anycatalyst or catalyst combination known to one skilled in the art, whichmay catalyse such a reaction. Suitable catalysts comprise at least oneGroup VIB metal component and at least one non-noble Group VIII metalcomponent selected from the group of iron, nickel or cobalt supported ona refractory oxide carrier. Examples of suitable Group IVB metals aremolybdenum (Mo) and tungsten (W). Examples of suitable non-noble GroupVIII metals are nickel (Ni) and cobalt (Co).

The refractory oxide support of the catalyst used in the firsthydrotreating step may be any inorganic oxide, alumino-silicate orcombination of these, optionally in combination with an inert bindermaterial. Examples of suitable commercially available hydrotreatingcatalysts are C-424, DN190, DN200 and DN3100 of Criterion CatalystCompany (Houston, Tex.).

The catalyst used in step (b) is suitably at least partly sulphidedprior to operation in order to increase its performance with time onstream. Presulphiding of the catalyst can be achieved by methods knownin the art, such as for instance those methods disclosed in the earlierreferred to publications relating to sulphided catalysts.

Step (b) is suitably operated at a temperature of between 250 and 420°C. and preferably between 350 and 400° C. The actual temperature willdepend largely on the content of sulphur and/or nitrogen in the feed andthe desired reduction to be achieved. Higher temperatures result inhigher reduction of S- and N-content. The pressure may range from 10 to250 bar, but preferably is between 20 and 150 bar. The weight hourlyspace velocity (WHSV) may range from 0.1 to 10 kg of oil per litre ofcatalyst per hour (kg/l.h) and suitably is in the range from 2 to 6kg/l.h

In step (c) the oil effluent of step (b) is contacted with a dewaxingcatalyst. The pour point of the oil is lowered to a value of between −9and −30° C. and more preferably to a value between −12 and −20° C. instep (c). This reduction can be achieved by for example adjusting theseverity of the reaction and the choice of the catalyst.

The dewaxing catalyst may be any catalyst, which is known to reduce thepour point of a hydrocarbon feed in the presence of hydrogen. Suitabledewaxing catalysts are heterogeneous catalysts comprising a molecularsieve and optionally in combination with a metal functionality having ahydrogenation function. Suitable metals are Group VIII metals, forexample nickel, cobalt, platinum and palladium. Combinations of platinumand palladium are also possible as well as combinations of nickel orcobalt with Group VIB metals, for example NiMo or NiW.

Molecular sieves, and more suitably intermediate pore size zeolites,have shown a good catalytic ability to reduce the pour point of a baseoil precursor fraction under catalytic dewaxing conditions. Preferablythe intermediate pore size zeolites have a pore diameter of between 0.35and 0.8 nm. Suitable intermediate pore size zeolites are ferrierite,ZSM-5, ZSM-12, ZSM-22, ZSM-23, SSZ-32, ZSM-35 and ZSM-48. ZSM-5 mayoptionally be used in its HZSM-5 form in the absence of any Group VIIIor VIB metals. The other molecular-sieves are preferably used incombination with the above listed metals. Further details and examplesof suitable catalysts and dewaxing conditions are for example describedin WO-A-9718278, U.S. Pat. No. 5,053,373, U.S. Pat. No. 5,252,527, U.S.Pat. No. 4,574,043, WO-A-0029511, WO-A-0029512 and EP-B-832171. Examplesof suitable commercial for bare or base metal dewaxing catalysts areZ-706, SDD-800 (as obtainable from Criterion Catalyst Company), Hydex-L(from Sud Chemie), HC-80 (from UOP) and the Mobil MLDW catalyst.Examples of noble metal based catalysts are Z-876A (Criterion CatalystCompany), the Mobil MSDW catalyst, ICR-410 (from Chevron) and DW-10(from UOP).

The effluent of step (b) may be directly used in step (c), for examplewhen at least steps (a)-(c) are performed in one staked bed reactorcomprising catalyst beds to perform the different steps. In such aseries-flow type of operation the level of organic bound nitrogen in theeffluent of step (b), which is used as feed to step (c), is preferablybelow 100 ppm and more preferably below 0.50 ppm. In the series flowembodiment the metal functionality of the dewaxing catalyst used in step(c) is preferably a non-noble metal from Group VIII, preferably nickel.The series flow embodiment is preferred because of its simplicity.

An alternative to the above series flow embodiment is an embodimentwherein hydrogen flow counter-current through a reactor in which steps(a)-(b) and optionally also steps (c) and optionally step (d) isperformed. In this embodiment the metal functionality of the dewaxingcatalyst is suitably a noble metal of or a combination of noble GroupVIII metals, preferably platinum optionally in combination withpalladium.

A next alternative to the series flow embodiment is a process whereinammonia and hydrogen sulphide are removed from the effluent of step (b)prior to feeding this effluent to step (c). This removal can be suitablyperformed by stripping the effluent with hydrogen. In this embodimentthe metal functionality of the dewaxing catalyst may be a noble metal ofor a combination of Group VIII metals, preferably platinum and/orpalladium. In this embodiment steps (c) and (d) are preferably performedin a counter current mode of operation. The conditions in step (c) areknown in the art and typically involve operating temperatures in therange of from 300 to 450° C., suitably from 330 to 400° C., hydrogenpressures in the range of from 10 to 200 bar, preferably from 40 to 150bar, weight hourly space velocities (WHSV) in the range of from 1 to 10kg of oil per litre of catalyst per hour (kg/l/hr), suitably from 2 to 6kg/l/hr and hydrogen to oil ratios in the range of from 100 to 2,000litres of hydrogen per litre of oil.

In step (d) a final hydrotreating step is performed mainly to saturateany unsaturated compounds, reduce the level of colour bodies andstabilize the oil. The hydro-treating catalyst used in step (d) may beone of the catalysts or catalyst combinations as described for step (b).Especially when steps (a)-(d) are performed in the above explainedseries flow a non-noble catalyst is used in step (d). When the dewaxingcatalyst of step (c) is based on a noble metal of Group VIII, thecatalyst of step (d) is preferably also based on a noble metal. Noblemetal based hydrotreating catalysts are suitably used at low hydrogensulphide partial pressures. Thus higher hydrogen partial pressures willfavour the use of such noble metal based hydrotreating catalysts. Suchhydro-treating catalysts suitably comprise a noble metal componentsupported on an amorphous refractory oxide carrier. Suitable noble GroupVIII metal components are platinum and palladium. Examples of suchcatalysts' are the commercially available C-622, C-624 and C-634 typesof Criterion Catalyst Company (Houston, Tex.). These platinum/palladiumcatalysts are advantageous because they deactivate less when the sulphurcontent of the feed to step (d) is still relatively high.

Suitably the same catalyst or catalyst combinations are used in steps(b) and (d). Step (d) is suitably operated at a temperature of between280 and 420° C. and preferably between 340 and 400° C. Highertemperatures result in higher reduction of the aromatic content in thehydrofinished product. The pressure may range from 10 to 250 bar, butpreferably is between 20 and 150 bar. The weight hourly space velocity(WHSV) may range from 0.1 to 30 kg of oil per litre of catalyst per hour(kg/l.h) and suitably is in the range from 10 to 20 kg/l.h.

The catalyst used in the different steps (a)-(d) may be a single typecatalyst or a combination or package of different catalyst having thesame functionality.

In a preferred embodiment of the invention steps (a) to (d) areperformed in one stacked-bed reactor as shown in FIG. 1. FIG. 1 shows areactor (1) provided with a feed inlet (2) to supply the oil andhydrogen to one or more beds (3) of hydrodemetallization catalyst orhydro-demetallisation catalysts combination in which step (a) isperformed. The reactor (1) is further provided with one or more beds (4)of a hydrotreating catalyst or hydrotreating catalysts combination inwhich step (b) is performed, one or more beds (5) of a dewaxing catalystin which step (c) is performed and one or more beds (6) in which step(d) is performed. Because step (a) is suitably performed at a highertemperature, suitably between 10 and 40° C. higher temperature, thanstep (b) a gas quench (7) is present, wherein via (8) an hydrogen-richstream can be supplied to the reaction mixture flowing through thereactor. The reactor is further provided with an outlet (9) for thefinal base oil product. The embodiment of FIG. 1 shows a process seriesflow configuration wherein hydrogen and the oil feed flow co-current.

The invention will be illustrated with the following non-limitingexamples.

EXAMPLE 1

A pre-processed used oil as obtained from the Interline reclaimingprocess was used as feed for this Example. The relevant properties ofthis feed are listed in Table 1. TABLE 1 Feed Sulphur (ppm) 5600Kinematic 6.7 cSt viscosity at 100° C. Nitrogen (ppm) 228 Viscosity 105Index Total metals (ppmw) 97 Pour Point  −7° C. Phosphor (ppmw) 31Boiling range: 355° C. IBP Calcium (ppmw) 32 50 vol % 450° C. Zinc(ppmw) 10 95 vol % 535° C. Silicon (ppmw) 10 Chlorine (ppmw) 50

The feed of Table 1 was fed to a stacked bed reactor as shown in FIG. 1.The upper catalyst bed consisted of Criterion hydrodemetallisationcatalyst RM-430, the second bed of Criterion hydrodemetallisationcatalyst RN-410, the third bed of Criterion hydrotreating catalystC-424, the fourth bed of commercial SDD-800 dewaxing catalyst and thefifth catalyst bed was again a Criterion C-424 hydrotreating catalyst.The operating pressure was 51.6 bar and the gas rate was 500 Nl/kg offeed. Further process conditions as in Table 2. TABLE 2 Step A1 A2 B C DCatalyst RM-430 RN-410 C-424 SDD-800 C-424 Temperature (° C.) 380 380365 365 365 WHSV (kg/l/hr)  12  12  4  4  15

The effluent of step (d) was distilled into 3 fractions: a fractionhaving a kinematic viscosity at 100° C. of 4.7 cSt, a fraction having akinematic viscosity at 100° C. of 9.35 cSt and a fraction boiling below370° C. The product fractions were analysed and the properties arelisted in Table 3. TABLE 3 Product Product fraction 1 fraction 2Kinematic viscosity at 4.7 9.35 100° C. (cSt) VI 96 102 Pour point −20−11 (° C.) Sulphur content mg/kg 259 535 Saturates content (wt % 77 73according to ASTM 2007) Metals content (ppmw) <1 <1 Chlorine content(ppmw) not not detectable detectable below below detection detectionlimit limit

The results listed in Table 3 show that starting from a pre-processedoil API Group I base oils having an improved pour point, close-to-zerometal and chlorine content and reduced content in sulphur and nitrogenis obtained in a high yield (97 wt % on 375° C.+ in feed) on feed whilethe viscosity index of each fraction is hardly affected. Anotherobservation is that the results have been obtained using the simpleseries flow embodiment as shown in FIG. 1. Thus a simple hydroprocessingmethod is provided to upgrade a pre-processed used oil to a base oilhaving properties comparable to those of virgin base oil.

EXAMPLE 2

Example 1 was repeated but at a process pressure of 121 bar and a gasrecycle rate of 1000 Nl/kg of feed. The base oil obtained as effluent ofstep (d) had the properties as listed in Table 4. In this case API groupII Base Oils have been obtained. TABLE 4 Product Product fraction 1fraction 2 Kinematic viscosity at 4.4 8.6 100° C. (cSt) VI 105 109 Pourpoint (° C.) −11 −7 Sulphur content mg/kg 9 20 Saturates content 93 91Metals content (ppmw) <1 <1 Chlorine content (ppmw) not not detectabledetectable

1. A process to further upgrade a pre-processed used lubricating oil by:(a) contacting the partially upgraded used oil in the presence ofhydrogen with a hydrodemetallization catalyst; (b) contacting theeffluent of step (a) in the presence of hydrogen with a hydrotreatingcatalyst; (c) contacting the effluent of step (b) in the presence ofhydrogen with a dewaxing catalyst; and (d) contacting the effluent ofstep (c) in the presence of hydrogen with a hydrotreating catalyst. 2.The process of claim 1, wherein the pre-processed oil has an oxygencontent of less than 1 wt %, a sulphur content of less than 2 wt % and achlorine content of between 10-300 ppm.
 3. The process of claim 2,wherein the pre-processed oil has an initial boiling point of between340 and 380° C.
 4. The process of claim 3, wherein step (a) is performedby first contacting the oil with two different types of ahydrode-metallisation catalyst, wherein the first has a higher metaluptake capacity than the second catalyst and the second catalyst has ahigher hydrodenitrogenation and hydrodesulphurisation performance thanthe first catalyst.
 5. The process of claim 4, wherein steps (a) to (d)are performed in series flow, such that gas and liquid flow co-currentwhen contacted with the catalysts, and wherein the reduction of organicbound nitrogen in steps (a) and (b) is such that the nitrogen content inthe effluent to step (c), is below 100 ppm.
 6. The process of claim 5,wherein the nitrogen content in the effluent to step (c) is below 50ppm.
 7. The process of claim 6, wherein the dewaxing catalyst used instep (c) comprises an intermediate pore size zeolite having a porediameter of between 0.35 and 0.8 nm and a non-noble Group VIII metal. 8.The process of claim 7, wherein the catalyst used in step (b) and (d)are the same catalyst comprising a Group VIB metal, a non-noble GroupVIII metal and a refractory oxide support.
 9. The process of claim 8,wherein the process is performed in one reactor comprising a number ofstacked beds of catalysts to perform steps (a) to (d).
 10. The processof claim 4, wherein at least steps (a) to (b) are performed in a reactorcomprising a number of stacked beds of catalysts to perform steps (a)and (b) and wherein gas and liquid flow counter-current when contactingthe catalyst.
 11. The process of claim 4, wherein hydrogen sulphide andammonia are removed from the effluent of step (b) before performing step(c).
 12. The process of claim 11, wherein the dewaxing catalyst used instep (c) comprises an intermediate pore size zeolite having a porediameter of between 0.35 and 0.8 nm and a noble Group VIII metal. 13.The process of claim 12, wherein the catalyst used in step (d) comprisesa noble Group VIII metal and a refractory oxide support.
 14. The processof claim 1, wherein the pre-processed oil has an initial boiling pointof between 340 and 380° C.
 15. The process of claim 1, wherein step (a)is performed by first contacting the oil with two different types of ahydrode-metallisation catalyst, wherein the first has a higher metaluptake capacity than the second catalyst and the second catalyst has ahigher hydrodenitrogenation and hydrodesulphurisation performance thanthe first catalyst.
 16. The process of claim 2, wherein step (a) isperformed by first contacting the oil with two different types of ahydrode-metallisation catalyst, wherein the first has a higher metaluptake capacity than the second catalyst and the second catalyst has ahigher hydrodenitrogenation and hydrodesulphurisation performance thanthe first catalyst.
 17. The process of claim 1, wherein steps (a) to (d)are performed in series flow, such that gas and liquid flow co-currentwhen contacted with the catalysts, and wherein the reduction of organicbound nitrogen in steps (a) and (b) is such that the nitrogen content inthe effluent to step (c), is below 100 ppm.
 18. The process of claim 2,wherein steps (a) to (d) are performed in series flow, such that gas andliquid flow co-current when contacted with the catalysts, and whereinthe reduction of organic bound nitrogen in steps (a) and (b) is suchthat the nitrogen content in the effluent to step (c), is below 100 ppm.19. The process of claim 3, wherein steps (a) to (d) are performed inseries flow, such that gas and liquid flow co-current when contactedwith the catalysts, and wherein the reduction of organic bound nitrogenin steps (a) and (b) is such that the nitrogen content in the effluentto step (c), is below 100 ppm.
 20. The process of claim 5, wherein thedewaxing catalyst used in step (c) comprises an intermediate pore sizezeolite having a pore diameter of between 0.35 and 0.8 nm and anon-noble Group VIII metal.
 21. The process of claim 5, wherein thecatalyst used in step (b) and (d) are the same catalyst comprising aGroup VIB metal, a non-noble Group VIII metal and a refractory oxidesupport.
 22. The process of claim 6, wherein the catalyst used in step(b) and (d) are the same catalyst comprising a Group VIB metal, anon-noble Group VIII metal and a refractory oxide support.
 23. Theprocess of claim 5, wherein the process is performed in one reactorcomprising a number of stacked beds of catalysts to perform steps (a) to(d).
 24. The process of claim 6, wherein the process is performed in onereactor comprising a number of stacked beds of catalysts to performsteps (a) to (d).
 25. The process of claim 7, wherein the process isperformed in one reactor comprising a number of stacked beds ofcatalysts to perform steps (a) to (d).
 26. The process of claim 1,wherein at least steps (a) to (b) are performed in a reactor comprisinga number of stacked beds of catalysts to perform steps (a) and (b) andwherein gas and liquid flow counter-current when contacting thecatalyst.
 27. The process of claim 2, wherein at least steps (a) to (b)are performed in a reactor comprising a number of stacked beds ofcatalysts to perform steps (a) and (b) and wherein gas and liquid flowcounter-current when contacting the catalyst.
 28. The process of claim3, wherein at least steps (a) to (b) are performed in a reactorcomprising a number of stacked beds of catalysts to perform steps (a)and (b) and wherein gas and liquid flow counter-current when contactingthe catalyst.
 29. The process of claim 10, wherein the dewaxing catalystused in step (c) comprises an intermediate pore size zeolite having apore diameter of between 0.35 and 0.8 nm and a noble Group VIII metal.30. The process of claim 10, wherein the catalyst used in step (d)comprises a noble Group VIII metal and a refractory oxide support. 31.The process of claim 1, wherein hydrogen sulphide and ammonia areremoved from the effluent of step (b) before performing step (c). 32.The process of claim 2, wherein hydrogen sulphide and ammonia areremoved from the effluent of step (b) before performing step (c). 33.The process of claim 3, wherein hydrogen sulphide and ammonia areremoved from the effluent of step (b) before performing step (c).